This invention generally relates to convex, three dimensional mirrors and, more particularly, to a mirror assembly, sometimes referred to as a "cross-over" mirror, which affords a bus driver, for example, a school bus driver, visual access in front of the school bus which is hidden from direct view as well as alongside the bus. Such cross-over mirrors can however also be used at the rear corners of a vehicle such as with trucks, mail vans and the like.
For many decades, cross-over mirrors and mirror assemblies have been deployed on school buses and are in fact required by federal and local regulations. A substantial body of prior art has been published describing various mirrors of the type to which the present invention relates. An exemplary list of such prior art includes U.S. Pat. Nos.: 4,822,157; 4,730,914; 4,436,372; 5,084,785; 5,589,984 and Des. 346,357. The above list represents but a fraction of the extensive prior art on the subject of cross-over mirrors and their accessories such as mounting hardware, mirror poles and other implements by which such mirror assemblies are secured to vehicles such as busses, school buses, trucks and the like. The contents of the aforementioned United States patents are incorporated by reference herein.
The convex, three-dimensional surface of the mirror lens described, for example, in the aforementioned U.S. Pat. No. 4,436,372, terminates in a continuous, peripheral edge which is essentially circular. That (and other similar) mirrors have a generally elliptical, i.e. dome, shape.
In more recent years, the prior art has moved to provide convex, three dimensional mirror lens surfaces that have a more stretched, elongate general shape. The aforementioned U.S. Pat. Nos. 4,822,157; 4,730,914; 4,436,372; 5,084,785; 5,589,984 and the Des. 346,357 illustrate the general style of such mirrors.
For the purposes of the present invention it is important to note that, essentially as a rule, the three dimensional, generally elliptical or convex surfaces of the aforementioned elongate cross-over mirror lens were provided with radii of curvature (measured along planar cross-sections) which were measurably non-constant, i.e. tending to increase or decrease from the center point on the mirror lens toward its peripheral, circumferential edge.
As an example, the convex, ellipsoid mirror lens shown in U.S. Pat. No. 4,436,372 has a generally flatter, i.e. less curved, center surface, which surface curves sharper as one proceeds toward the peripheral edge.
Stated differently, the "radius of curvature" of the surface decreases from the center where the major and minor axis of the mirror surface intersect toward the peripheral edge of the mirror. A similar relationship is specifically claimed for the elongate, oval mirror that is described in the aforementioned U.S. Pat. No. 5,589,984. In another patent, an opposite relationship is specified--the sharpest curvature is at the center, as the mirror surface flattens out as one proceeds toward the peripheral edge. In the mirror lens of the U.S. Pat. No. 4,730,914, the inventors stress the fact that the mirror surface has a generally constant radius of curvature at a central portion of the mirror representing about one half of the entire surface and different radii of curvature at the other portions of the mirror lens.
In part, the present inventor perceives that the prior art was constrained by the type of technology commonly used in the industry for forming a mirror lens, which technology inherently imparts non-uniform radii of curvature to the mirror lens. Indeed, at least some people adhered to a conventional wisdom that it is desirable to vary the radius of curvature so as to obtain a larger and less distorted image at the mirror center, but a greater field of view, through the provision of a more distorted image, at the peripheral regions on the mirror. The idea is to increase the space that the mirror monitors in and around the school bus or the like.